This invention relates to electrical machines and more particularly to those having a superconducting winding on the rotor cooled by a cryogenic liquid coolant, such as helium, which becomes a vapor in the operation of the machine.
Superconducting generators of early design were not capable of remaining superconducting throughout the uncleared fault (short circuit) interval required in modern power grid systems. Present estimates of the required faultworthiness of a superconducting generator to provide power system stability range up to about 15 or more cycles (1/4 second or more) of critical fault clearing time. During a fault, heat is generated in the rotor's cold electromagnetic shield and superconducting winding which may produce a pressure rise in the rotor of about 3 to 4 atmospheres. Vapor cooled leads, such as those of Gamble Pat. No. 4,091,298, May 23, 1978, carry current from the warm collector ring (or other external current source) to the cryogenic zone with a low amount of heat leakage, because cold vapor produced in the rotor flows counter to the direction of heat flow from room temperature. Such leads are thus designed to take care of the required external connections to the superconducting winding and also the needs for vapor removal from the liquid cooled rotor. Under steady state conditions, the vapor pressure drop from the cold end to the warm end of the lead may be only about 0.1 atmosphere and the cold end heat leak approximately 1.2 W/K Amp for each lead, with about 1 cm.sup.2 /K Amp of copper area of the conductor over its length.
When a fault occurs and rotor vapor pressure necessarily rises, cold vapor flow through the lead from the rotor increases in response to the increased pressure differential, causing a surge in vapor flow. The surge flow causes the smallish copper area lead and helium outlet temperature to drop very rapidly. The flow can be reduced to some extent by using an outlet orifice at approximately room temperature. Simple orifices do not control the excess flow well because the density of the surge vapor stream increases with temperature drop and mass flow is proportional to the square root of pressure difference and density. Without adequate control of the flow, the surge stream can cause icing of the collector rings and chilling of the rotating seals below their operating temperature limit, which leads to failure. It is with the objective of insuring faultworthiness, while not having to alter presently used arrangements of collector rings and rotating seals, that the present invention came about.
By the present invention, the outlet temperature of the surge stream is controlled at the warm end of the conductive lead by a "regenerator" portion of the lead that warms the surge stream and stabilizes its temperature and density. By the term "regenerator" in this context is meant a thermally massive element that, when required, transfers thermally stored heat to a gas stream in like fashion as a blast furnace regenerator. Stabilizing the density of the vapor therefore stabilizes the mass flow. The regenerator portion of the lead has a thermal mass (equal to its density.times.its specific heat.times.the metal volume thereof) that is much larger, preferably about an order of magnitude or more, than the total thermal mass of the surge stream itself (equal to its density.times.its specific heat.times.its total volume flow in a fault interval). In addition, the cooling effectiveness of the regenerator portion (equal to its actual heat transfer divided by its maximum theoretical heat transfer) is in excess of 99%. These characteristics of the regenerator are sufficient to assure that the helium return stream is warmed before it returns through the rotating transfer seals.
The design of the regenerator portion may be accomplished by the use of a block of conductive material that has a thermal mass substantially greater than that of the transferred vapor, including that transferred during fault conditions, such as by at least an order of magnitude. This may be accomplished by utilizing as the regenerator portion a bus conductor, such as of copper, of a cross-sectional area of conductive material at least an order of magnitude greater, and an overall volume at least two times greater, than that of the conductive element of the conductor that is at the cold end. For heat transfer between the regenerator portion and the vapor being transferred, the bus conductor includes a plurality of vapor transfer passages within the conductive material.